Fuel cell power plants are well known and are commonly used to produce electrical energy from hydrogen containing reducing fluid fuel and oxygen containing oxidant reactant streams to power electrical apparatus such as power plants and transportation vehicles. In fuel cell power plants of the prior art, it is known that product water generated by fuel cells of the plant is often managed and removed through porous water transport plates of a coolant system, as well as through evaporation and/or entrainment within a cathode exhaust stream. Such a coolant system is also utilized to provide water through the porous water transport plates to humidify fuel and oxidant reactant streams to thereby minimize drying out of proton exchange membrane (“PEM”) electrolytes as well as to remove condensate which occurs within the cell. Porous water transport plates when filled with a liquid also provide gaseous seals to prohibit mixing of the reactant oxidant and fuel streams.
Fuel cell power plants having porous water transport plates, however, give rise to water management difficulties. Total water volume in such plants is very high. Such a high water volume requires complex and costly plant systems, especially when the plant is operated in sub-freezing ambient conditions. For example, mechanical damage may occur when the product water and/or any water coolant fluid freezes without elaborate freeze protection apparatus.
Solutions to such freezing related problems are described in U.S. Pat. No 6,528,194 B1 and U.S. Pat. No. 6,562,503 B2 both of which are entitled “Freeze Tolerant Fuel Cell Power Plant”, and both of which are owned by the owner of all rights in the present invention. Those patents disclose the use of water immiscible fluids, as purge fluids, during a shutdown and startup of the plant to displace water from key system components.
When the fuel cell power plant disclosed in those patents is shut down for a short term shut down, displacement valves operate to control flow of the water coolant into a freeze tolerant, open tube accumulator, and the water immiscible fluid into the coolant loop to displace the water coolant to the accumulator. For a long term shut down, the same procedure is undertaken to direct the water coolant into the accumulator; to direct the water immiscible fluid into the coolant loop to displace the water coolant; and, to then drain the water immiscible fluid back into the accumulator.
To start up the power plant after a long term shut down, the water immiscible fluid is first directed to pass from the accumulator through a heater and a re-cycle line to flow through open tubes of the accumulator to melt the frozen water coolant. Whenever fuel cells of the plant have attained a desired operating temperature and the water coolant within the accumulator has thawed, flow of the water immiscible fluid out of the accumulator is terminated, and thawed water coolant is directed to flow through the coolant loop to cool plant fuel cells and manage fuel cell product water. The freeze tolerant fuel cell power plant is then in a steady-state operation wherein the water coolant continues to cycle from the accumulator through the water transport plate of the coolant loop, and back to the accumulator. The displacement or purge of the water coolant by the water immiscible fluid out of the fuel cells and coolant loop prevents mechanical damage to the plant by preventing the freezing of the water coolant during a shutdown and startup, until the water coolant is within the accumulator. Also, the low freezing temperature water immiscible fluid transfers heat from the fuel cells or an external heater to melt frozen coolant water within the accumulator upon start up.
Nonetheless, the prior art has limitations. The prior art fails to rectify the high volume of water in the fuel cell power plant. The high volume of water is a result of two related but separate plant requirements, of first cooling operating fuel cells, and second, managing water within the plant, including fuel cell product water. Consequently, there is a need for a fuel cell power plant that may be efficiently operated in sub-freezing conditions with effective water management and a minimal volume of water.